Investigations have shown that the anterior cruciate ligament (ACL) is rarely lax and experiences the greatest tension at full-extension. If an ACL is torn, or if for some other reason the knee encounters an anterior instability, the knee requires support at full-extension. Conventional knee braces provide support to the tibial condyles by applying a force with a static strap or a rigid frame.
Application of a force with a static strap or a rigid frame has several disadvantages. First, the static strap and rigid frame spread the force over a relatively large amount of generally soft tissue. Second, if the static strap or rigid frame were to produce a sustained, concentrated force on the tibial tuberosity, it would be extremely uncomfortable for the wearer over time. Consequently, the wearer would likely loosen the straps and thereby eliminate the necessary supportive force.
Moreover, studies have shown that the posterior cruciate ligament (PCL) prevents posterior displacement of the tibia on the femur and prevents hyperextension at the knee joint. These studies also indicate that the PCL is taut at full-extension, becomes progressively more lax until 30 degrees of flexion, and thereafter becomes increasingly more tense until it reaches a maximum tension at 130 degrees of flexion. When the PCL is torn, a posterior instability can result that produces a greater susceptibility to further weaken or tear the PCL due to hyperextension.
Typical knee braces attempt to prevent hyperextension through range of motion stops. It is difficult, however, to prevent hyperextension using range of motion stops because of the soft tissue in and around the knee. The range of motion stops stop the rigid frame from rotating, but the soft tissue in the knee can give, allowing hyperextension.